Conventional Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM) bit cells include a transistor and a magnetic tunnel junction (MTJ) structure. The basic MTJ structure consists of two magnetic electrodes sandwiching an oxide tunnel barrier layer. The magnetic moment of each magnetic electrode is oriented along a long axis of a laterally elongated element. The parallel and anti-parallel magnetic moment orientation between the two magnetic layers on either side of the tunnel barrier gives rise to two different resistances across the barrier, resulting in two memory states. One of the magnetic electrodes, referred to as the free layer, has a magnetic moment direction that can be switched. The other magnetic electrode, referred to as a reference layer, has a magnetization that is pinned to a particular direction.
In conventional STT-MRAM structures, an injected current becomes spin polarized due to fixed magnetization in the reference layer, resulting in a spin transfer torque (STT) at the magnetization of the free layer. When the current density of the injected current exceeds a threshold, the magnetization orientation of the free layer can be switched by the spin transfer torque. The resulting memory state (i.e., 0 or 1) is determined by the direction of the current.
Conventionally, both a read process and a write process are operated by use of current injection. For the write process, the current density allowed to flow through the MTJ device in an STT-MRAM bit cell has a strong dependence on the MTJ resistance, such that when the MTJ resistance is lower, more current is allowed to pass through the MTJ device. Therefore, a smaller MTJ resistance provides a larger data writing operation margin for MRAM designers. However, the smaller MTJ resistance also results in smaller data read sensing margins. Likewise, a larger MTJ resistance that improves data read sensing margins impairs data writing operation margins. As a result, the MTJ resistance of conventional MTJ designs represents a design compromise between improving data read sensing margins and improving data writing operation margins.
Further, dual MTJ STT-MRAM cell designs have been proposed to increase write capability of the MTJ cell. However, the dual MTJ designs place even further limitations on MTJ resistance and a greater sensitivity to data read sensing margins.